PULMONATA. 9 



Those which have attained their extreme dimensions and weight 

 are too heavy to trust themselves to so frail a support. They 

 have no power to elevate themselves again, and in this respect 

 are inferior to the spiders, which can both lower and raise them- 

 selves by the aid of the secreted thread. Like the spiders, how- 

 ever, they often remain suspended in mid-air for a time, and it is 

 not unlikely that there is some pleasurable sensation connected 

 with the act, which induces them thus to prolong it. We have 

 seen the descent actually practised by every one of our Atlantic 

 species. 



Besides the v/atery fluid which at all times lubricates the in- 

 teguments, the animals can, at their will, secrete at any point, or 

 over the whole surface of their bodies, a more viscid and tenacious 

 mucus than is usually exuded. This power is used as a means 

 of def-ence. Whenever a foreign substance touches them, imme- 

 diately a quantity of this mucus, of the consistence of milk and 

 nearly of the same color, is poured out and forms a kind of mem- 

 brane interposed between themselves and the irritating substance. 

 So, also, when they are surrounded by a corrosive gas, or are 

 thrown into water or alcohol, they form over themselves in this 

 way a thick protecting covering, which is undoubtedly a non- 

 conductor of heat and impervious, at least for a time, to liquids. 

 Shielded by this coating, they can live the greater part of a day 

 immersed in water, and for a shorter time in alcohol ; and M. 

 Ferussac asserts that they have survived for hours in boiling 

 water. They leave a trace of their usual secretion on every 

 object over which tliey pass, and thus can easily be traced to their 

 retreats. The ordinary secretion is most abundant at their 

 posterior extremity. The secretion of the mucous fluid over 

 their surface is necessary to their existence. Death immediately 

 follows the failure of this power, and is preceded by the drying 

 up of the skin. 



All the species are extremely voracious, and devour an in- 

 credible quantity of food in a short time. Those found in this 

 country are generally supposed to be vegetable feeders, but 

 nearly all of them subsist occasionally upon dead animal matter, 

 of which they seem to be fond, and when in confinement some- 

 times attack and devour each other : and the foreign genus, 

 Testacella, is known to prey habitually upon earth-worms It 

 is probable, therefore, that in their natural condition, all of them 



